1. Field of the Invention
The present invention relates to a cellulose powder of spherical, non-fibrillar cellulose particles and a process for the production of this.
2. Description of Related Art
The applications for cellulose powder are diverse and range from the pharmaceutical sector to the construction industry. Cellulose powders can be found in the laboratory as a chromatography column filler as well as in large-scale technical processes as auxiliary filter materials. In the food stuff and cosmetics industry they are widely used as auxiliary and carrier substances. In general a trend is noticeable that there are attempts to replace inorganic powders (e.g. of silica) with cellulose powders (O. Schmidt, “washing filtration of wine: with kieselgur or cellulose”, The German Wine Magazine, pp. 29-35, 2004).
Cellulose powders are mainly obtained by preparing and grinding pulp or even wood and annual plants. As a result of an additional hydrolysis step, so-called microcrystalline cellulose (MCC) can be obtained. FIG. 1 shows the production of cellulose powders out of cellulose in diagram form in accordance with the state of the art.
Depending on the complexity of production, powders of different qualities (purity, size, etc.) are obtained. What all these powders have in common is that they reveal a fibrous structure since natural cellulose has a pronounced tendency in any order of magnitude to form fibrous structures like this. Likewise MCC, which is also described as spherical, is consisting of crystalline fiber fractions.
Depending on their microstructure, structure types of the cellulose can be differentiated, in particular cellulose-I and cellulose-II. The differences between these structural types can be determined by radiography and they are described in detail in the scientific literature.
All of the powders described above are consisting of cellulose I. There are a large number of patents for the production and application of cellulose-I-powders. Numerous technical details are also patented in these e.g. with regard to grinding. As described above, cellulose-I-powders are of a fibrous nature which is a hindrance for a number of applications. Thus, for example, suspensions of powders of this kind have a viscosity which depends to a considerable extent on the shear strain. In the same way fibrous powders often display a limited pourability since the fibers can get caught up.
Cellulose powders on the basis of cellulose-II can be found on the market. Cellulose powders with this structure can either be obtained from a solution (mostly viscose) or via the disintegration of cellulose-II-powders (e.g. cellophane). In the same way very fine powders are procurable but only in small quantities (10 μm and less).
U.S. Pat. No. 5,244,734 describes the production of spherical particles with diameters of lower than 20 μm from a viscose dispersion.
WO 99/36604 describes fibrous particles which are obtained from the regeneration of viscose with the highest possible shear strain. For example a stream of 500 ml/min of a diluted viscose solution with a cellulose concentration of 1 to 4 weight percentage is led through a 3 mm nozzle into a stirred precipitation bath. This results in fibrous, fibrillar particles due to the high shear strain and the low cellulose concentration.
WO 2007/003699 describes the production of cellulose particles as a result of the regeneration of a cellulose solution in an acid (e.g. by spraying or mixing) with a particle size of preferably 0.05-10 μm. The production of cellulose solution is performed by the enzymatic treatment of the pulp followed by a reaction with an alkali. The cellulose particles are used as a filling material and coating pigment for paper since they are light and eco-compatible, as a cellulosic powder for the food stuff industry as well as for the pharmaceutical industry. One variant described is the modification of cellulose before or after regeneration e.g. via acetylation and the increase in porosity by adding substances which dissolve in these regeneration conditions.
WO 2007/003697 describes the coating of cellulose particles (0.05-10 μm) with substances which disperse light (e.g. silicates, titanium oxide). These particles are likewise used as a filling material and may be useful as a coating pigment for paper and carton.
According to WO 2006/034837, cellulose particles with a length of preferably 50 μm and a thickness of 1 μm are produced in water by exerting shear forces on a suspension of a starting product containing cellulose. These are to be used as a ceramic aggregate for paper, carton and coating colors to increase the tenacity, improve the print image, increase the color intensity and as a carrier material for other substances. These cellulose particles are fibrous and/or fibrillar.
The production of cellulosic micro particles for cosmetic applications via the incorporation of powder-like substances in viscose can be found in JP 63092603. In this respect a dispersion is formed of viscose, inorganic filling material and a water-soluble anionic, high molecular weight substance. The coagulation is, for example, produced by heating. There are no precise details about the size of the cellulose particles obtained.
WO 93/13937 describes the production of cellulose balls with a diameter of preferably lower than 25 μm with a dense surface which reveals no pores, scratches or anything similar: first of all an emulsion is made of viscose in oil. The coagulation takes place for example in water-free, alcoholic salt solutions. Finally washing takes place with concentrated salt solutions containing alcohol to prevent swelling and then this is regenerated with acid solutions containing alcohol. The particles produced in this way are applied in chromatography particularly as carrier materials, so-called carriers for ligands (enzymes, antibodies, peptides, proteins). Due to the dense surface of the ball-shaped cellulose particles, no interfering diffusion processes occur.
In U.S. Pat. No. 5,196,527 the production of porous, ion-exchanging, ball-shaped, e.g., oval, cellulose particles with a size of 3-400 μm is described using the following steps: first of all coagulated viscose particles are made by forming a dispersion which contains cellulose xanthate and finally coagulating the viscose parts as a result of heating or with the help of a coagulation reagent. These viscose parts are regenerated by an acid or via a crosslinking followed by regeneration. As a result of the crosslinking, a high pressure resistance is attained in the particles. In a final step, ion-exchanging groups are introduced to the cellulose molecule. These cellulose particles have a cellulose-II-structure and an x-ray crystallinity of 5-50%, and preferably 20-40%.
These particles are also used in chromatography as well as for cleaning and separating different substances. If the cellulose particles are to be used as carriers, then certain ligands are introduced instead of ion-exchanging groups e.g., antibodies to separate antigens and/or peptides to separate enzymes.
U.S. Pat. No. 5,026,841 deals with the production of porous, ball-shaped cellulose particles with a preferred particle size of 5-80 μm. To this end a cellulose ester of an aromatic carboxylic acid, which is dissolved in an organic solvent, is introduced to an aqueous solution of an anionic tenside while stirring vigorously, the organic solvent is removed, the parts are isolated and finally the carboxylic acid groups are separated. These particles are used as filling materials, carriers for filtration, cleaning, adsorption, carriers, to set inorganic or organic materials as well as in the form of carriers for enzyme and for protein immobilization and cleaning.
KR 950000687 describes the production of a cellulose powder (1-20 μm) for cosmetic products of Ramie fiber according to the following process: Treatment with alkaline hypochlorite solution, washing, dispersing in water and finally spray drying.
JP 2006028452 describes the production of a cellulose solution as a pouring aid for powder-shaped food stuffs e.g., common salt, whereby 50% of the parts possess a particle size of between 1-50 μm.
According to U.S. Pat. No. 4,415,124, a micro powder of cellulose is produced by reinforcing/condensing, followed by grinding (at least 90% of the parts with a particle size of lower than 125 μm). The particles received are used as an additive for mortar, plaster masses, wallpaper paste, filling material for molding mass and plastics as well as in the form of carrier materials in the pharmaceutical industry.
EP 1693402 describes, among other things, the production of a cellulosic body with a preferred size of 1 μm-5 mm, preferably 10 μm-500 μm of porous cellulosic parts (also regenerated). In one variant, cellulosic parts are distributed in an alkaline medium which contains a binder (e.g., waterglass). The suspension is converted in droplet form and finally coagulated in a corresponding coagulation solution whereby the cellulose particles join so that intermediate spaces are left. These particles are used as carriers to mobilize enzymes, as carriers or a matrix for adsorption or as cosmetic additives. The advantages are the high mechanical resistance and the high throughput rates which are possible.
U.S. Pat. No. 4,659,494 describes the production of a powder to clean carpets which contains cellulose powder with a particle size of preferably 5-50 μm as an adsorption agent. No more details are given of the production of the cellulose powder but it would appear to be obtains as a result of disintegrating pulp.
Viscose processes which proceed via the intermediate stage of cellulose xanthate are not desirable nowadays due to the manifold environmental burdens, among other things, with sulphurous waste gases which require complex waste gas purification in their production.
Starting from viscose technology WO 02/00771 and U.S. Pat. No. 6,174,358 utilize NMMO as an alternative solvent for the production of cellulose particles. However, both patents deal with fibrillar and not spherical particles.
According to WO 02/00771 cellulose particles are made by dispersing a cellulose solution (of cellulose or cellulose derivatives, solvent acetone, N-methylmorpholine-N-oxide, aqueous caustic lye of soda, etc.) in a precipitation medium. Additives such as cationization agents or filling materials may also be contained in the solution. Dispersing is done via the shear stress produced with the help of a refiner.
DE 19755353 describes a similar method whereby a solution of cellulose in NMMO is formed to a jet which is divided into individual segments by means of rotating cutting beams. The solution parts are intercepted in a dispersion medium. The pearl-shaped cellulose parts are then received by cooling down the dispersion or by precipitating with a precipitation agent. The process is used to produce porous pearl celluloses with a particle size of 50-1000 μm which are used as separation and carrier materials for medical and diagnostic purposes, as adsorber materials e.g., for blood detoxification and as cell culture carriers.
DE 10221537 describes the production of hybridic molded bodies with a size of 1-1000 μm in which polysaccharide solutions containing additives are dispersed in an inert solvent so that solvent drops form. This is preferably cellulose dissolved in N-methyl morpholine-N-oxide-monohydrate. The additives are for example ceramic powders. Finally the dispersion developed from this is cooled down whereby the solvent drops solidify. These are separated off and introduced to a precipitating medium. Drying then takes place and/or a thermal treatment.
WO 02/57319 describes the production of cellulose pearls using the NMMO process whereby different additives in large quantities are added to the cellulose solution prior to forming, for example titanium oxide or barium sulphate and materials acting as ion exchangers. The products obtained can be used as ion exchangers or catalysts.
The production of a titanium oxide suitable as an ion exchanger for example for waste water purification is described in U.S. Pat. No. 6,919,029. Particularly high absorption capacities and speeds are achieved with this material since the titanium oxide material is activated via a special treatment on the surface. This titanium oxide material can be described as a “substoichiometric titanium oxide”. This means that the ratio of the oxygen atoms to the titanium atoms in the material is smaller than 2. Reference is made to the description in U.S. Pat. No. 6,919,029 for a more detailed description of this surface activation.
Other possibilities to produce titanium oxides which are particularly well suited to waste water treatment, are in the so-called “doping” of titanium oxide with iron and sulphur atoms. These substances have a photocatalytic activity.
All of these processes have the disadvantage that an orientation of the cellulose molecules develops in the resulting particles when exposing the cellulose solution to only very slight shear forces which, following complete coagulation or regeneration, leads to a fibrillar, i.e., fibrous particle structure. The demand for spherical instead of fibrillar powders is, however, constantly on the increase since fibrillar powders reveal various disadvantages. Thus fibrillar powders are not suitable for some applications (e.g., viscosity modifiers) because they change the flow behavior of the liquid, to which they are added, so that it is dependent on the shear strain, i.e. in the direction of non-Newtonian behavior. This is not desirable with printing inks, for example. Moreover, spherical cellulose powders flow more readily and can be more easily conveyed, dosed and dispersed. This plays an important role particularly with applications in the pharmaceutical, cosmetics and foodstuff industry and when replacing silica gel and other inorganic substances. However, these particles must have a sufficient absorption capacity for the different substances despite the non-fibrillar structure e.g., pharmaceutical active agents, aromas, oily and fatty substances.